Methacrylic resins are used in a wide range of indoor and outdoor applications because they are excellent in weatherability, rigidity, and molding appearance. However, since methacrylic resins are brittle polymers in themselves, there are cases where they arise troubles due to their insufficient impact resistance according to the ways in which they are used.
Investigations on improvements in the impact resistance of methacrylic resins have been made from long ago. Known techniques for the improvements include: a method in which an acrylic rubber having a multilayer structure is blended with a methacrylic resin (Examined Japanese Patent Publication No. 55-27576, Unexamined Published Japanese Patent Application No. 3-199213, and Examined Japanese Patent Publications Nos. 58-1694, 59-36645, 59-36646, 63-8983, and 62-41241); a method in which monomers containing methyl methacrylate as the main component are graft-polymerized with a conjugated diene polymer (Examined Japanese Patent Publication No. 46-18491 and Unexamined Published Japanese Patent Applications Nos. 55-147514 and 55-123612); and a method in which monomers containing methyl methacrylate as the main component are graft-polymerized with a rubber-like polymer obtained from either a copolymer of an aromatic vinyl compound and a conjugated diene or a conjugated diene polymer by hydrogenating at least 70%, substantially at least 90%, of the olefinically unsaturated bonds, i.e., double bonds, thereof (Unexamined Published Japanese Patent Applications Nos. 1-297413, 2-302459, and 3-295643).
The first method, in which an acrylic rubber having a multilayer structure is blended with a methacrylic resin, is the technique currently most extensively used industrially. The multilayered acrylic rubber has a spherical structure made up of three or more layers comprising, stacked substantially alternately, rigid layers made mainly from methyl methacrylate and flexible layers made mainly from an acrylic ester such as butyl acrylate. The degree of crosslinking of the flexible layers, in particular, is controlled by changing polyfunctional monomers. A feature of this blending technique resides in that since the flexible layers have been sufficiently crosslinked, the rubber particles are neither deformed nor broken by the mechanical shearing force which the particles receive during extrusion or injection molding. Although this method is an excellent technique for improving a performance, i.e., impact resistance, of methacrylic resins, it has problems including the following. Since the rubber ingredient has a multilayer structure containing rigid layers, it is necessary to add the rubber ingredient in a large amount for obtaining the desired impact resistance. Moreover, since multistage polymerization should be conducted repeatedly in forming the multilayer structure, the production efficiency is low, resulting in an increased cost.
The second method, in which monomers containing methyl methacrylate as the main component are graft-polymerized with a conjugated diene polymer, is characterized in that since the rubber ingredient contains a large amount of double bonds, it undergoes crosslinking reactions like the rubber ingredients in ABS resins and high-impact polystyrenes and, hence, the rubber particles are neither deformed nor broken by the mechanical shearing force which the particles receive during extrusion or injection molding. In addition, the second method is excellent also in cost because the production efficiency is high. However, this technique has problems, for example, in that the rubber ingredient contains a large amount of double bonds which are unstable to light. As a result weatherability inherent in methacrylic resins is reduced considerably and, hence, the final resin cannot be used in outdoor applications.
The third method, in which monomers containing methyl methacrylate as the main component are graft-polymerized with a rubber-like polymer obtained by hydrogenating either a copolymer of an aromatic vinyl compound and a conjugated diene or a conjugated diene polymer, is not always superior to the second method in that when the rubber ingredient used is a hydrogenated copolymer of an aromatic vinyl compound and a conjugated diene, the final resin has poor weatherability because it undergoes yellowing, etc. in outdoor use due to the aromatic vinyl compound. On the other hand, in the case where a hydrogenated conjugated diene polymer is used as the rubber ingredient, the final resin tends to have improved weatherability because the rubber ingredient has a smaller amount of double bonds as compared with that in the second method. However, since at least 70%, substantially at least 90%, of the double bonds in the conjugated diene polymer have been hydrogenated, namely, since the rubber has a reduced amount of double bonds, the crosslinking reaction of the rubber ingredient does not proceed and, hence, the rubber particles are deformed or broken by the mechanical shearing force which the particles receive during extrusion or injection molding. As a result, this technique has problems, for example, in that molded articles usually have a poor surface gloss and are apt to have an appearance similar to that of ground glass according to molding conditions, or that the resin gives a molded article in which the gate part differs from the front part in surface gloss and which hence has poor evenness in gloss. Thus, this third method is not a practically usable technique for improving the impact resistance of methacrylic resins required to have an attractive surface.
Many proposals such as those described above have been made in order to impart impact resistance to methacrylic resins. However, no technique for improving impact resistance has been found with which an impact-resistant methacrylic resin having the excellent properties inherent in methacrylic resins can be produced highly efficiently at low cost.
An object of the present invention is to provide a methacrylic resin having excellent impact resistance which is industrially more usable in general purposes, retains the excellent weatherability and surface gloss inherent in methacrylic resins, and is optionally excellent in transparency and other properties.